Quaternion.Angle

public static float Angle (Quaternion a, Quaternion b);

Description

Returns the angle in degrees between two rotations a and b.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
    public Transform target;
    void Update() {
        float angle = Quaternion.Angle(transform.rotation, target.rotation);
    }
}